The use of self-crosslinking vinyl ester dispersions in the production of non-wovens is known. Self-crosslinking polymeric binders increase the wet and dry strength on mechanical load and improve the resistance to water and solvents during cleaning. The crosslinking agents employed in practice are predominantly monomers containing N-methylol groups, such as N-methylol drivatives of unsaturated organic acid amides (N-methylolacrylamide) or ethers thereof (N-(iso-butoxyumethyl)-acrylamide). When these compounds are used as crosslinking agents, free methylol groups are present in the dispersion or are formed by hydrolysis of the derivatives in aqueous medium. Formaldehyde is eliminated from N-methylol compounds in aqueous media, but the equilibrium is far over towards the intact methylol group. Aqueous dispersions of self-crosslinking copolymers containing N-methylol groups, therefore, always contain formaldehyde, even if in only small amounts. As a consequence of the toxicological doubts regarding formaldehyde, which have been discussed for some time, and the regulation that only formaldehyde-free plastic dispersion systems may be used for non-wovens in the sanitary and hygiene areas, there is a necessity to reduce the formaldehyde content in self-crosslinking polymer dispersions or to make available formaldehyde-free self-crosslinking polymer dispersions.
Various ways of reducing the formaldehyde content or preparing formaldehyde-free binders for non-wovens are known from the specialized literature.
DE-Al 3,202,122 (U.S.A. 4,476,182) describes formaldehyde-free acrylate dispersions having hydroxyl and carboxyl groups. Although the fiber non-wovens strengthened using these systems have good mechanical values, the resistance towards organic solvents is, however, not sufficient, meaning the crosslinking agents, such as, for example, glyoxal, must be added in order to achieve good stability during cleaning.
In DE-Al 3,328,456 (EP-Al 143,175), formaldehyde-free, crosslinking polymer systems containing crosslinking components based on N-methylolamide and/or N-methylolether amide groups are claimed. The formaldehyde reduction is achieved here by adding a formaldehyde acceptor based on cyclic ureas, such as, for example, ethyleneurea, which bonds the free formaldehyde produced. The disadvantage of this procedure is that the wet strength values, in particular, of the bound non-wovens are reduced by adding water-soluble organic substances, and formaldehyde is still present, although in bound form, and may be liberated, for example, on heating.
A route which is analogous to DE-Al 3,328,456 and has the abovementioned disadvantages is used in EP-Bl 80,635. Here, urea as formaldehyde scavenger is added to the dispersion.
A further process for reducing the content of the free formaldehyde in the binder dispersion is claimed in EP-A3 121,864 (USA 4,449,978). Here, the formaldehyde emission is reduced by replacing N-methylolacrylamide units by acrylamide units. Formaldehyde-free dispersions cannot be obtained using this procedure, but above all, the strength properties and the resistance during cleaning of the non-wovens treated with this binder are greatly reduced.
Formaldehyde-free acrylate dispersions are claimed in EP-A2 193,107. Derivatives of acrylamidoglycolic acid as crosslinking components are copolymerized here with (meth)acrylates. Although the fiber non-wovens strengthened using these dispersions are distinguished by high wet strength and by high water and washing lye resistance, the resistance to organic solvents is, however, unsatisfactory--it is necessary to introduce additional crosslinking agents into the dispersion.
EP-A2 184,153 describes formaldehyde-free binders, for non-wovens, based on copolymers containing unsaturated dicarboxylic acids and (meth)acrylamide as crosslinkable comonomers. Due to the absence of self-crosslinkability, the fiber non-wovens strengthened therewith have inadequate mechanical strength values and poor solvent resistance.
The processes described show that although it is, in priciple, possible to provide formaldehyde-free or formaldehyde-reduced binder systems for strengthening non-wovens, the strength values and, in particular, the solvent resistance, above all in the case of complete substitution, have not yet reached the level of binder systems containing N-methylol units.
The object was, therefore, to develop, as binders for non-wovens, crosslinkable, aqueous copolymer dispersions, above all containing vinyl esters having a greatly reduced content of free formaldehyde or containing no free formaldehyde and imparting good mechanical values and solvent resistances on the strengthened fiber non-wovens.